All-glass, ophthalmic lenses have relatively high densities which may result in sufficient weight to cause wearer discomfort. Organic plastic lenses, having relatively low densities, and hence less weight, have become popular. However, these are generally less scratch-resistant than glass lenses.
In recent years, photochromic lenses, that is, reversibly darkening glass lenses, have become popular. As yet, however, organic photochromic materials have not been developed that are resistant to fatigue, that is, loss of the reversibly darkening characteristic with use. Therefore, a strong desire has existed to develop a lighter weight, ophthalmic lens having the potential for photochromic behavior.
My U.S. Pat. No. 4,793,703 describes a three-layer composite lens. The structure is composed of an inorganic glass layer, which may be photochromic, a layer of a rigid organic plastic, and an interlayer of a flexible, organic adhesive that can be cured in the vicinity of room temperature. Characteristically, the linear coefficient of expansion of the glass is about 60-120.times.10.sup.-7 /.degree.C., and that of the rigid plastic layer is about 200-700.times.10.sup.-7 /.degree.C.
This construction alleviated severe stress conditions, and consequent delamination, that had been encountered with higher expansion plastics, such as CR-39 commonly used in organic lenses. It also enhanced optical clarity, particularly under high humidity conditions.
Nevertheless, attempts to develop a commercial product demonstrated a need for further improving the bond strength between the layers. A particularly annoying problem was the tendency for a laminated lens to undergo delayed delamination after exposure to high humidity or boiling water. Such delamination of a lens occurs upon standing for a matter of days after testing positive in the aqueous environment to determine resistance to environmental conditions.